The invention relates generally to the field of power conversion devices and more particularly to inverters and their control.
Large number of topographies and types of power conversion circuits are known and are in use. Many of these circuits rely upon inverter topologies for converting direct current (DC) power to control frequency alternating current (AC) power. In many topologies a rectifier or other converter is provided to receive incoming AC power, typically from the grid, and to convert the AC power to DC power that is applied to a DC bus used to feed the inverter circuitry. Such topologies are used in a variety of applications, such as for controlling the speed and operating characteristics of motors.
Motor drives utilizing inverter topologies often employ a single converter and single inverter coupled to one another by a single DC bus. Conventional inverters are formed by solid state switches provided in pairs and alternately switched between conducting and non-conducting states to provide desired output waveforms, typically of controlled frequency. Such topologies are adequate for many smaller applications, and may vary in size depending upon the power rating, frame size, voltage, and other specifications of the driven motor. However, for larger motors the components of such drives become proportionally large and expensive. It becomes attractive, then, to use alternative topologies in which multiple inverters are provided in parallel, with their outputs being joined to provide a common AC output to a load.
Such parallel inverter applications pose unique difficulties. For example, because the outputs of the inverters are essentially shorted to one another, various magnetic structures may be required to prevent circulating currents from being established due to mismatched timing in the switching of the power electronic switches in the inverters. For example, if switches in a first inverter are switched in a manner that is not sufficiently synchronized with switches of a parallel inverter, output power can be allowed to re-enter one or the other of the inverters due to the conductive states of the switches. Because the switching frequencies of such inverter components is quite high, a high degree of precision will be required to prevent such circulating currents absent magnetic structures of the type used in the art. Furthermore, techniques for maintaining synchronous operation of parallel inverters may be more efficiently and accurately conducted in parallel. Sufficient topologies for precise switching and parallel synchronizing techniques have yet to be proposed.